tokfandomcom-20200215-history
Volcanic arcs
A volcanic arc is a chain of es formed above a , positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic . Generally, volcanic arcs result from the of an oceanic under another tectonic plate, and often parallel an . The oceanic plate is saturated with water, and volatiles such as water drastically lower the melting point of the . As the oceanic plate is subducted, it is subjected to greater and greater pressures with increasing depth. This pressure squeezes water out of the plate and introduces it to the mantle. Here the mantle melts and forms at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone. These should not be confused with volcanic chains, where volcanoes often form one after another in the middle of a tectonic plate, as the plate moves over the hotspot, and so the volcanoes progress in age from one end of the chain to the other. The form a typical hotspot chain; the older islands (tens of millions of years old) to the northwest are smaller and have more soil than the recently created (400,000 years ago) Hawaii island itself, which is more rocky. Hotspot volcanoes are also known as "intra-plate" volcanoes, and the islands they create are known as Volcanic Ocean Islands. Volcanic arcs do not generally exhibit such a simple age-pattern. There are two types of volcanic arcs: * oceanic arcs form when subducts beneath other oceanic crust on an adjacent plate, creating a volcanic island arc. (Not all are volcanic island arcs.) * s form when oceanic crust subducts beneath on an adjacent plate, creating an arc-shaped . In some situations, a single subduction zone may show both aspects along its length, as part of a plate subducts beneath a continent and part beneath adjacent oceanic crust. Volcanoes are present in almost any mountain belt, but this does not make it a volcanic arc. Often there are isolated, but impressively huge volcanoes in a mountain belt. For instance, and the volcanoes in Italy are part of separate but different kinds of mountainous volcanic ensembles. The active front of a volcanic arc is the belt where develops at a given time. Active fronts may move over time (millions of years), changing their distance from the as well as their width. Petrology In a , loss of from the subducted induces partial melting of the overriding and generates low-density, that buoyantly rises to and be extruded through the of the overriding plate. This loss of water is due to the destabilization of the mineral at approximately 40–60 km depth. This is the reason for island arc volcanism at consistent distances from the subducting slab: because the temperature-pressure conditions for volcanism due to chlorite destabilization will always occur at the same depth, the distance from the trench to the arc volcanoes is determined only by the dip angle of the subducting slab. On the subducting side of the arc is a deep and narrow , which is the trace at the Earth's surface of the boundary between the down-going and overriding plates. This trench is created by the gravitational pull of the relatively dense subducting plate pulling the leading edge of the plate downward. Multiple s occur along this subduction boundary with the s located at increasing depth under the island arc: these quakes define the s. The volcanic arc forms when the subducting plate reaches a depth of about . Ocean basins that are being reduced by subduction are called 'remnant oceans' as they will slowly be shrunken out of existence and crushed in the subsequent collision. This process has happened over and over in the geologic history of the Earth. In the rock record, volcanic arcs can be seen as the volcanic rocks themselves, but because volcanic rock is easily and , it is more typical that they are seen as s, the rocks that formed underneath the arc (e.g. the ), or in the sedimentary record as s. Image:Cascadia subduction zone USGS.png| Cascade Volcanic Arc, a continental volcanic arc Image:map_of_alaska_volcanoes_okmok.jpg| The Aleutian Arc, with both oceanic and continental parts. Examples Two classic examples of oceanic island arcs are the in the western and the in the western . The in western and the along the western edge of are examples of continental volcanic arcs. The best examples of volcanic arcs with both sets of characteristics are in the North Pacific, with the Aleutian Arc consisting of the and their extension the on the , and the Kuril-Kamchatka Arc comprising the and southern . Continental arcs * * and * * ** ** ** ** * Island arcs * * * * including the * : ** ** ** * * * and * and * * ** * and * * * Egean, or * , including the * : ** * Ancient island arcs * * * References Category:Earth